The present invention relates to a safety valve for a tank or a pipe for pressurized fluid, intended to be opened at a predetermined temperature in order to release the pressurized fluid and the opening of which is triggered by the melting of a meltable material.
Such safety valves are used to prevent the risk of the tank exploding when its temperature exceeds a predetermined limit. This is because, at this temperature, which may be of the order of 100xc2x0 C., the meltable material, which may be a meltable metal, melts and makes it possible for the gases and the excess pressure to escape. Below this critical temperature, the meltable metal should normally remain solid and prevent any leak of pressure and of gas.
Unfortunately, it has been noticed that it happens that the meltable metal is prematurely, that is to say at temperatures less than its melting temperature, subject to plastic flow. It has been noticed that these cases occur when the gas is stored at high pressures and that the origin of this plastic flow is the combination of temperature and pressure. It has in fact been found that if such a meltable metal is exposed for a relatively long time to high pressures, it may deform and thus release gases below the melting temperature. There is therefore an undesired leak with risk of accident and risk for the environment. Furthermore, the material of the meltable metal to be replaced is excessively expensive.
Document EP 1069355 describes such a safety valve which comprises a plug made of meltable material intended to melt at a predetermined temperature in order to release the pressurized gas of a tank. The meltable plug is subject to the action of a differential action sliding piston intended to reduce at least some of the effect of the gas pressure on the meltable plug to prevent the plug melting at a temperature less than the predetermined temperature under the effect of the pressure.
The aim of the present invention is to provide a novel safety valve which also makes it possible to extract the meltable material at the gas pressure and which requires a smaller amount of meltable material than the known valves.
To achieve this objective, the present invention provides a valve of the type described in the preamble, which is characterized in that it comprises a body in the shape of a socket, the open end of which is exposed to the gas pressure and the bottom of which is closed except for a minuscule orifice, in that the side wall of the body comprises at least one radial opening, in that the body is engaged, by its bottom, by sliding, in a cap of complementary shape and is fastened thereto by means of the meltable material, in that the said cap comprises at least one access to the atmosphere, in that the cap, after melting of the meltable material, can be moved axially with respect to the body until each access communicates with a side opening in the body.
The cap preferably comprises a stop which limits the relative movement between the cap and the body in the position where the windows are released.
Thus, in the closed position of the valve, when the meltable metal is solid, the latter is subject only to a very small force through the orifice at the bottom of the socket. This force is not enough to adversely affect the physical state of the meltable material. On the other hand, when the cap is exposed to a temperature exceeding the melting point of the metal, the latter melts and releases the cap from the body. The gas pressure, which is exerted through the orifice at the bottom of the socket, is then enough to move the cap with respect to the body until it is in the position where the side openings and the windows are aligned and through which the gas can escape.
Moreover, the invention has the advantage of requiring only very little meltable metal. In fact, it is enough to provide a thin film between the bottom of the cap and the bottom of the body in order to form a weld thereon and to fasten these two elements to each other.